我们所看到的历史的范围和边界是被历代史家划定的，他们没有框进来的内容，我们就看不到了。

单项选择题Ametalrod with length Lis pushedalong a set of conducting rails thatcompletesa circuit with a total resistance Rata constant velocity vtothe right. The circuit is in a magnetic field Bthatpoints out ofthepage.Whatis the current induced in the circuit? A、RBLv clockwise B、BLv Rcounterclockwise C、BLv R clockwise D、BLv R counterclockwise E、BLv Rclockwise

单项选择题发球区的宽度为( )(单选题) A、无限长 B、9米 C、8米